<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>文件上传</title>
    <style>
        #preview{
            filter: contrast(7);
        }
    </style>
</head>

<body>
    <input value="13" disabled/>
    <script>
        var a=('1.23.45.6'.split('.')).length-1;
        console.log(a);
    </script>
</body>

</html>